1. Field of the Invention
This invention relates to the purification of water, and more particularly concerns compositions for the selective removal of trace levels of dissolved metals from water.
2. Description of the Prior Art
Undesirable pollutant species such as dissolved toxic metals can occur regularly or periodically in natural bodies of water such as lakes, rivers, streams, groundwater and stormwater, and in industrial waters such as landfill leachate, municipal sewage systems and wastewater discharges. The toxic metals are primarily multivalent “heavy” metals, and can be caused to form insoluble compounds with anionic additives. The most prevalent technique for the removal of toxic heavy metals is to add to the water a sufficient amount of an anionic precipitating agent. The resultant precipitate is then allowed to settle or is removed by filtration or other methods.
Although precipitative techniques are effective, they require that the water undergoing remediation be accumulated in tanks and held there for considerable periods of time with agitation while the treatment agents are added and dispersed, causing the heavy metals to form precipitates of sufficiently large particle size to facilitate settling and/or filtration. Operations of this nature require large, expensive equipment installation and high operating expense. The high cost of such installations is justifiable only where extremely large volumes of water must be treated on a long term basis, such as in municipal waste water treatment facilities and operations involving large industrial facilities.
Another general technique for removing dissolved metal species from water involves the passage of a stream of the water undergoing remediation through a substantially stationary, water permeable bed of absorbent material. Suitable absorbent materials for such on-the-run treatments include ion exchange resins having affinity for species in either cationic or anionic form. The advantage of such technique is that the water can be treated while flowing through a confined bed of the resins, thereby avoiding the need for large holding and processing tanks.
Ion exchange resins are generally in the form of spherical beads having been made by catalyzed suspension polymerization of a liquid styrene/divinylbenzene prepolymer mixture. By virtue of controlled conditions of mixing and use of surfactants, the prepolymer is suspended as discrete spherical droplets within an engulfing continuous phase aqueous medium. The resultant polymer product is in the form of spherical beads having a size generally in the range of 16 to 50 mesh (U.S. Sieve Series), or 1.2 mm to 0.3 mm, respectively. The beads are then subjected to chemical treatments which impart specific ion exchange functionality. Because of their bead form and relatively large diameter, beds of ion exchange resins in vertically oriented columns or tanks offer relatively little impedance to the flow of water through the bed.
Other specialized absorbents for the removal of trace pollutants from water are available, in the form of irregularly shaped granules, the most notable example being granular activated carbon. When comparing absorbents in bead form with absorbents in granule form at the same particle size, beds of granular form absorbents produce higher flow impedance. Even higher flow impedance is encountered when the granules are comprised of water swollen soft polymer, usually referred to as hydrogel polymer and containing between 30% and 70% by weight of water. Examples of the use of hydrogel granules for the selective removal of trace heavy metals from water are disclosed in U.S. Pat. Nos. 7,041,222; 3,715,339 and elsewhere. The expression “trace heavy metals” as used herein is intended to denote waters having less than 50 parts per million (ppm) of a targeted metal ion.
The aforesaid hydrogel polymer is generally produced by the thermal condensation polymerization of an aqueous prepolymer solution. The resultant polymer is produced as irregularly shaped granules in the size range of 10 to 80 mesh (U.S. Sieve Series). Although the granules, when employed as an absorption bed, cause high impedance to the flow of water through the bed, they possess two valuable characteristics, namely: 1) a degree of selectivity that permits removal of specific toxic heavy metals in the midst of hundred-fold greater concentrations of commonly abundant innocuous species, and 2) extremely high kinetics, namely the ability to remove selective species at 90% efficiency within contact times less than one minute.
Allowed U.S. patent application Ser. No. 11/803,076, filed May 14, 2007 discloses the blending of metal-absorbing hydrogel granules with spacer particles to produce a composition which, when confined as a bed within a conduit through which water flows, provides little impedance to the flow of water and resists fouling by accumulation of suspended particles in the water undergoing treatment. Said compositions, however are not shape-retaining structures. Because of this, it has been found that the spacer particles, whether of fibrous or non-fibrous configuration, separate from the hydrogel granules by gravimetric action and differential flotative effects. Difficulties are also encountered in emplacing said compositions within a conduit to form a bed of uniform consistency.
It is accordingly an object of the present invention to diminish the impedance to flow of beds of polymeric absorbent in granular form used for removing dissolved metal species from water flowing through said bed.
It is another object of this invention to dispose metal-absorbing granules within a porous shape-retaining structure which provides low resistance to the flow of water therethrough.
It is a further object of the present invention to provide a shape-retaining structure of the aforesaid nature confining metal-absorbing granules and which is easily emplaced within a water-carrying conduit.
It is a still further object of this invention to provide a process for producing the aforesaid shape-retaining structure having confined granules for the absorption of dissolved metals.
These objects and other objects and advantages of the invention will be apparent from the following description.